The present invention relates to the process control industry. More specifically, the present invention relates to an isolation diaphragm or seal, of the type used to couple a process control instrument to a process.
Some types of process control instruments, such as pressure transmitters, have a pressure sensor which is fluidically coupled to an isolation diaphragm by a fill fluid. The isolation diaphragm comprises part of a subassembly called a "remote seal" or a "diaphragm seal" and isolates the pressure sensor from corrosive process fluids being sensed. Pressure is transferred from the isolation diaphragm to the sensor through the fill fluid which is substantially incompressible and fills cavities on both sides and a capillary tube (or thru-hole if the seal is directly mounted to the instrument). The tube is typically flexible and may extend for several meters. The process medium contacts the remote isolation diaphragm which conveys the exerted pressure to the pressure sensor disposed in the transmitter housing.
Typically, the isolation diaphragm and any process-watted parts of the remote seal are made of a corrosion resistant material such that the process medium does not damage the diaphragm. However, the housing of the remote seal may partially contact the process medium as well. Therefore, it is necessary that the housing of the remote seal be protected from contact with the process medium or otherwise be corrosion resistant. The prior art has attempted to protect the housing of the remote seal using a number of techniques.
One type of prior art seal is known as an "extended seal" and includes a stainless steel tube welded to a stainless steel flange. A Hastelloy end cap which carries a Hastelloy diaphragm is welded to the stainless steel tube opposite the flange. A tubular sheet of Hastelloy fits over the stainless steel tube and is welded to the end cap. A disc of Hastelloy is banded to the flange and to the tubular sheet and covers the flange. The Hastelloy disc and the tubular sheet typically have a thickness of between 1/64 and 3/16 inches. However, such an extended seal requires sufficient room to accommodate the tube and may not be used in all installations.